Recently, linear mono-pole antennas or folded mono-pole antennas have been used, in general, as mobile antennas for mobile radio devices. Those conventional antennas are described hereinafter with reference to FIGS. 6A and 6B. A conventional mono-pole antenna shown in FIG. 6A comprises planar conductive ground plate (or grand plane) 91 made of copper, power feed point placed at the center of ground plate 91, and antenna element 93 shaped like a wire or a rod and made of copper. Element 93 has a height of “H” in a vertical direction with respect to ground plate 91, and its first end P1 is coupled to power feed point while its second end P2 is open.
FIG. 6B shows a conventional folded mono-pole antenna 100, which includes antenna element 103 shaped like “square C” formed by double-backing a copper wire or a copper rod. Element 103 has a height of “H” vertically with respect to conductive ground plate 91, and is folded at height “H” to form “square C”. Element 103 has a first end P1 coupled to power feed point and a second end P2 coupled to ground plate 91.
In the construction discussed above, feed of a high frequency current of an operating frequency from signal source via power feed point 92 to antenna element 93 (103) of antenna 90 (100) excites antenna element 93 (103) for transmission. On the other hand, in the case of reception, a high frequency electromagnetic field of the operating frequency excites antenna element 93 (103) for reception.
Since antenna element 93 of mono-pole antenna 90 has the first end P1 coupled to power feed point 92 and the second end P2 open at the height of “H” vertically from ground plate 91, current (i1) between points “P1” and “P2” and in-phase image current (i1) corresponding to points “P1” and “P2” flow to ground plate 91. As a result, element 93 is excited, thereby radiating radio-wave into the air.
On the other hand, folded mono-pole antenna 100 has element 103 folded into a shape of “square C”, so that current (i1) between points “P1” and “P2” and current (i3) between points “P3” and “P4” as well as in-phase image currents (i1, i3) corresponding to points “P1” and “P2” and points “P3” and “P4” flow to ground plate 91. As a result, the impedance of antenna 100 increases, thereby broadening its available frequency band.
A folded antenna is disclosed in, e.g. Japanese Patent Unexamined Publication No. S62-122401.
The foregoing conventional antennas work in a ¼ wavelength mode, so that mechanical height “H” needs to be approx. a ¼ wavelength. For instance, an antenna of car telephones, which use 810 MHz–958 MHz (hereinafter referred to as PDC800) band, needs a height of approx. 83 mm.
If height “H” of an antenna element is shortened to a height lower than a ¼ wavelength of the operating frequency, the antenna impedance becomes smaller and it is difficult to obtain an impedance matching. If the foregoing conventional antenna is placed at a rear tray or a dashboard in a car, the antenna is preferably installed such that element 93 (103) is oriented upward; however, the upward installation allows element 93 (103) to occupy a large space in a height direction. As a result, these types of antennas are obliged to limit a mounting place of the antenna or a design of a car body.